Asymmetric operation method of non-volatile memory structure

ABSTRACT

An operation method for a non-volatile memory structure formed between two doping regions serving as bit lines in a semiconductor substrate, the non-volatile memory structure comprising a first conductive line serving as a select gate and being formed above the semiconductor substrate, two conductive blocks serving as floating gates and being formed at the two sides of the first conductive line and insulated from the first conductive line with two first dielectric spacers therebetween, a first dielectric layer formed on the two second conductive blocks, a second conductive line serving as a word line and being formed on the first dielectric layer and substantially perpendicular to the two doping regions. While reading the programmed status of one of the conductive blocks, a bias voltage is applied to the doping region next to the conductive block to be read, a bias voltage is applied to the second conductive line, and a bias voltage is applied to the first conductive line next to the conductive block to be read, so as to turn on the select gate and form an inversion layer underneath the select gate; and the doping region, the channel under the conductive block and the inversion layer under the select gate form a reading path during the reading operation.

BACKGROUND OF THE INVENTION

(A) Field of the Invention

The present invention is related to an operation method of a non-volatile memory structure and, more particularly, to an asymmetrical operation method of a self-aligned non-volatile memory structure with high cell density.

(B) Description of the Related Art

Erasable programmable read only memories (EPROMs) and electrically erasable programmable read only memories (EEPROMs) are well known non-volatile memory devices capable of being stored data, as well as erased and rewritten data, as desired.

The conventional non-volatile memory cells normally need high currents to operate, e.g., 200 microamperes (μA), for hot electron programming, so it is not suitable for low-power devices that are in the trend of chip development. Therefore, one form of an EEPROM device includes a so-called “split-gate” electrode has been developed to obtain high efficiency and low current programming, where the programming current can be diminished to, for example, 1 μA.

Many EEPROM devices use two polysilicon layers (one for the formation of the floating gate and another for the formation of the control gate and possibly electrical interconnects) whereas other EEPROM devices use three polysilicon layers. For example, U.S. Pat. No. 4,302,766 provides a first polysilicon layer serving as the floating gate, a second polysilicon layer serving as the control gate, and a third polysilicon layer coupled through an erase window to a portion of the first polysilicon layer for use during erasure of the cell. U.S. Pat. No. 4,331,968 also uses a third layer of polysilicon to form an erase gate, and U.S. Pat. Nos. 4,561,004 and 4,803,529 use three polysilicon layers in their own specific configurations.

U.S. Pat. No. 4,622,656 describes an EEPROM device in which a reduced programming voltage is provided by having a highly doped channel region under the select gate, and having the channel region under the floating gate being either lightly doped or doped to the opposite conductivity type, thereby providing a significant surface potential gap at the transition location of the channel.

Moreover, some recently developed innovative memory structures with accompanying figures are introduced in detail as follows.

U.S. Pat. No. 5,712,180 discloses a flash EEPROM cell layout as shown in FIG. 1(a), and the cross-sectional diagram of line A-A in FIG. 1(a) is shown in FIG. 1(b). EEPROM cell 101 includes a buried source region 102 and a buried drain region 103, each being buried by a relatively thick layer of dielectric 104 and 105, respectively. Channel region 106 is divided into a first portion 106-1 and a second portion 106-2. The first portion 106-1 is influenced by the polysilicon layer 109 and serves as a select gate, whereas the second portion 106-2 is influenced by a floating gate 107 formed of a polysilicon layer and which, in turn, is influenced by control gate 108 formed of a polysilicon layer. As is well known in the art, suitable dielectric layers such as thermally grown oxide are located between channel 106, polysilicon layer 109, and polysilicon layer 107 for insulation. Similarly, suitable dielectric layers such as oxide or composite oxide/nitride are formed between the three layers of polysilicon. Metal silicide can be used in place of one or more of the polysilicon layers 108 and 109. If desired, a highly-doped P⁺ region 120 is used within channel 106-2 adjacent to buried drain region 103, so as to provide a stable threshold voltage of the memory transistor including channel 106-2. Accordingly, the floating gate 107 has to be larger than the polysilicon layer 109 in width direction, i.e., the longitudinal direction shown in FIG. 1(a) for alignment tolerance consideration. Therefore, it is hard to decrease the scale of this type of structure.

U.S. Pat. No. 5,414,693 also disclosed a flash EEPROM memory structure as shown in FIG. 2, and the cross-sectional diagram of line B-B in FIG. 2 is shown in FIG. 3. The cell structure is formed in a P-doped substrate 206 with a drain 204 of a transistor 201 and a drain 205 of a transistor 202. The drains 204 and 205 serve as bit lines. The transistor 201 includes a floating gate 207 a and an overlying control gate 208 a. Likewise, the transistor 202 includes a floating gate 207 b and a control gate 208 b located thereon. A word line 209 extends over the two transistors 201 and 202 and forms the control gate 209 a of the select gate between the floating gate transistors 201 and 202. The word line 209 serially connects the select gates in one row of a memory array and runs perpendicular to the bit lines in columns, i.e., the drains 204 and 205. Similarly, to allow alignment tolerance, floating gates 207 a and 207 b need to be larger than the select gate 209 a in width direction, i.e., the longitudinal direction as shown in FIG. 2. Therefore, the memory cell size is difficult to be decreased significantly more.

To sum up, the floating gates in U.S. Pat. Nos. 5,712,180 and 5,414,693 are completely defined by the control gates during control gate patterning, so the floating gates have to be larger than the select gate in width direction to allow alignment tolerance. Under such circumstances, the memory cell size could not be reduced significantly.

SUMMARY OF THE INVENTIION

An aspect of the present invention is to provide an operation method of a memory structure for low power device applications, and this memory structure comprises competitive small memory cells so that the cell density can be increased effectively.

An operation method of a memory structure, for example, including two cells formed between two doping regions, in a semiconductor substrate is disclosed. The memory structure essentially comprises a first conductive line, two conductive blocks, two first dielectric spacers, a first dielectric layer, and a second conductive line. The first conductive line, e.g., a polysilicon line, is formed above the semiconductor substrate, and the two conductive blocks, for example, composed of polysilicon are formed at the two sides of the first conductive line and insulated from the first conductive line with the two first dielectric spacers. The first dielectric layer such as an oxide/nitride/oxide (ONO) layer is formed on the two second conductive blocks and above the first conductive line, and the second conductive line is formed on the first dielectric layer and is substantially perpendicular to the two doping regions.

Accordingly, the stack of the conductive block, the first dielectric layer, and the second conductive line form a typical floating gate structure, i.e., the conductive block can store charges. The first conductive line and conductive blocks function as a select gate and floating gates, respectively, whereas the doping regions and the second conductive line function as bit lines and a word line, respectively. In addition, the first conductive line may also serve as an erase gate for data erasure.

While reading the programmed status of one of the conductive blocks, a bias voltage is applied to the doping region next to the conductive block to be read, a bias voltage is applied to the second conductive line serving as the word line, and a bias voltage is applied to the first conductive line serving as the select gate next to the conductive block to be read, so as to turn on the select gate and form an inversion layer underneath the select gate, and the doping region, the channel under the conductive block to be read and the inversion layer under the select gate form a reading path during the reading operation.

Moreover, the conductive block can be programmed by putting a bias voltage on the doping region next to the conductive block to be programmed, a bias voltage on the word line, and a bias voltage on the first conductive line next to the conductive block to be programmed, so as to turn on the select gate and form an inversion layer underneath the select gate, and the doping region, the channel under the conductive block to be programmed and the inversion layer under the select gate form a programming path during the programming operation.

The above split gate memory structure can be manufactured by the following steps. First of all, two fourth conductive lines are formed above a semiconductor substrate, and two first dielectric spacers are formed on both sides of each fourth conductive line. Then, dopants are implanted into the semiconductor substrate to form two doping regions next to the two fourth conductive lines, and a first conductive line is formed between the two fourth conductive lines. The first conductive line can be formed by depositing conductive material and planarizing it afterwards. The two fourth conductive lines are etched to form two conductive blocks next to the first conductive line. Then, the first dielectric layer is formed on the two conductive blocks and above the first conductive line, and a second conductive line perpendicular to the doping regions is formed on the first dielectric layer.

Further, with a view to increasing the word line integrity, the following process may be added. A dielectric layer is deposited and etched so as to form two second dielectric spacers beside the second conductive line. The second dielectric spacers can be formed by oxidation also. Then, a third conductive line substantially parallel to the second conductive line is formed, where the third conductive line serves as another word line and the second dielectric spacer function as an insulator between them, and a dielectric, e.g., oxide or ONO, is located between the third conductive line and the semiconductor substrate. As a result, the word line density will be almost doubled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) illustrate a known memory structure;

FIGS. 2 and 3 illustrate another known memory structure;

FIGS. 4(a) through 4(i) illustrate the processes of manufacturing the memory structure in accordance with the present invention;

FIG. 5 illustrates the circuit schematic with reference to the memory structure in accordance with the present invention;

FIG. 6 illustrates the top view of the memory structure in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described with reference to the accompanying drawings.

A process for making a memory cell of the NMOS type is exemplified as follows with a view to illustrating the features of the present invention.

As shown in FIG. 4(a), a gate dielectric layer 402 ranging from 70 to 110 angstroms is thermally grown on the surface of a semiconductor substrate 401, and followed by sequentially depositing a conductive layer 403 and a mask layer 404 thereon. The conductive layer 403 may be composed of polysilicon and have a thickness between 500-1000 angstroms, and the mask layer 404 may be a silicon nitride layer of a thickness between 200-1000 angstroms.

Sequentially, the gate dielectric layer 402, the conductive layer 403, and the mask layer 404 are patterned by lithography and etching so as to form a plurality of conductive lines 403.

In FIG. 4(b), two dielectric spacers 405 ranging from 100 to 300 angstroms are formed beside each conductive line 403, and then photoresist blocks 406 are formed and N⁺ dopants, e.g., arsenic ions with 1×10¹⁴−5×10¹⁵ atoms/cm² are implanted into the semiconductor substrate 401 that are uncovered by the photoresist blocks 406, so as to form N⁺ regions 407. The dielectric spacers 405 can be formed by either oxidation or deposition of low pressure chemical vapor deposition (LPCVD) oxide, or high temperature oxide (HTO), and followed by etching back. Then, the photoresist blocks 406 are stripped away.

In FIG. 4(c), dielectric layers 408 and 409 are formed on the semiconductor substrate 401, in which the dielectric layers 408 and 409 are disposed alternately. The dielectric layers 409 are formed on the doped regions 407, so that the thicknesses thereof are thicker than those of the dielectric layers 408 under oxidation. Alternatively, the dielectric layers 408 and 409 can be either oxide or ONO layers. Sequentially, a conductive layer, such as a polysilicon or polysilicon/tungsten silicide (poly/WSi) layer 410, is deposited thereon.

In FIG. 4(d), the conductive layer 410 is planarized by either chemical mechanical polishing (CMP) or etching back to be a plurality of conductive lines 410. Then, dielectric layers 411 are formed on the top of the conductive lines 410. The dielectric layers 411 can be formed by either CVD oxide deposition followed by etching back or by thermal growth, i.e., oxidation.

In FIG. 4(e), the mask layer 404 is removed by, for example, phosphoric acid, and then a dielectric layer such as an ONO layer 412 is formed along the contour of the device. Afterwards, another conductive layer 413 is deposited thereon.

FIG. 4(f) illustrates the top view of the device shown in FIG. 4(e). The conductive layer 413 is etched to form separated conductive lines 413 serving as word lines, and at the same time the conductive lines 403 are separated into a plurality of conductive blocks. As a result, the stack of the conductive block 403, the ONO layer 412 and the conductive line 413 serve as a floating gate structure, i.e., the conductive block 403 is used for storage. Then, CVD oxide is deposited and planarized to form isolating lines 414 between the conductive blocks 403 and between the conductive lines 413.

FIG. 4(g) illustrates the cross-sectional view of line 1-1 in FIG. 4(f), where the nitride layer 416 is used as a mask on the conductive lines 413 for the planarization to the CVD oxide while the isolating lines 414 are being formed.

As shown in FIGS. 4(h) and 4(i), FIG. 4(i) is the cross-sectional view of line 2-2 in FIG. 4(h). Alternatively, dielectric spacers 417 can be formed beside the conductive lines 413, and then conductive lines 418, also serving as word lines, are formed between the dielectric spacers 417 so that, ideally, the density of the word lines can be doubled.

FIG. 5 illustrates a schematic diagram with reference to the split gate memory structure put forth in the present invention, in which the memory cell architecture is the same as that shown in FIG. 4(e). Yet some components are renamed by their functionality: a data line (bit line) is denoted by DL_(x), a select gate is denoted by SG_(x), and a word line is denoted by WL_(x). Floating gate cells are denoted by FG_(x), where the floating gate cells at two sides of a select gate SG₂ are denoted by T_(L) and T_(R), respectively. Examples for the reading, programming, and erasing of memory cells T_(L) and T_(R) are shown in Table 1.

For reading T_(L), WL₂ and SG₂ are 5V, and the DL₂ is 1.2V, where the 5V on SG₂ will form an inversion layer underneath the select gate SG₂, and the inversion layer will be connected to input/output circuit at a voltage around 0V. Thus, bit line DL₂, the channel underneath T_(L), and the inversion layer under the SG₂ will form a reading path during reading operation.

For programming T_(L), WL₂ and SG₂ are 12V and 5V, respectively, and the DL₂ is 5V, where the 5V on SG₂ will form an inversion layer underneath the select gate SG₂, and the inversion layer will be connected to input/output circuit at a voltage around 0V. Consequently, 5V bias is generated underneath the floating gate FG₁. Therefore, electrons will jump into the storage cell of T_(L) for programming.

As such, the inverse layer underneath the select gate can be deemed a bit line of high resistance, such that the reading and programming operations can be controlled from one side of the conductive block, i.e., asymmetrical operation. Therefore, the operation will be simple and can be well controlled.

For erasing T_(L), as shown in Erase (I) of Table 1, a highly negative voltage such as −18V is applied to WL₂ to expel electrons out of floating gate FG₁ and into the semiconductor substrate through the dielectric layer underneath. To avoid a very high negative voltage which causes complex circuit design, a manner by partitioning voltage can be employed as shown in Erase (II). For instance, DL₁, DL₂, and DL₃ are 5V, and WL₂ is −10V. Therefore approximately −5V will be coupled to the FG₁ in the case of a 50% coupling ratio. Therefore, 10V bias is generated across the FG₁. Furthermore, the dielectric spacer 405 may function also as a tunnel oxide, and the conductive line 410, such as SG₂, may function as an erase gate. Such erase conditions are listed in Erase (III).

The programming, reading and erasing of T_(R) is substantially the same as that of T_(L), so that the detailed description is omitted herein. TABLE 1 SG₁ SG₂ SG₃ WL₁ WL₂ WL₃ DL₁ DL₂ DL₃ T_(L) Read 0 V 5 V 0 V 0 V  5 V 0 V 0 V 1.2 V   0 V Program 0 V 5 V 0 V 0 V  12 V 0 V 0 V 5 V floating Erase (I) 0 V 0 V 0 V 0 V −18 V 0 V 0 V 0 V 0 V Erase (II) 0 V 0 V 0 V 0 V −10 V 0 V 5 V 5 V 5 V Erase (III) 0 V 12 V  0 V 0 V  0 V 0 V 0 V 0 V 0 V T_(R) Read 0 V 5 V 0 V 0 V  5 V 0 V 0 V 0 V 1.2 V   Program 0 V 5 V 0 V 0 V  12 V 0 V 0 V floating 5 V Erase (I) 0 V 0 V 0 V 0 V −18 V 0 V 0 V 0 V 0 V Erase (II) 0 V 0 V 0 V 0 V −10 V 0 V 5 V 5 V 5 V Erase (III) 0 V 12 V  0 V 0 V  0 V 0 V 0 V 0 V 0 V

FIG. 6 is a plan view of the memory layout with reference to FIG. 5 specifying the relationship of the floating gates FG₁, FG₂, and FG₃, select gates SG₂ and SG₃, data lines DL₂ and DL₃, and word line WL. The floating gates are totally self-aligned to the select gates and word line so that there is no need for alignment tolerance at all. Consequently, a very competitive small size can be achieved, thereby significantly increasing the cell density of the memory structure.

FIG. 6 also shows one of the methods to connect an inversion layer under the select gate to an outside circuit, in which an N⁺ diffusion region was formed and overlapped with an end of the select gate and a metal layer contacts the N⁺ diffusion region. Thus, a bias voltage will be transferred from the metal layer to the N⁺ diffusion region and then to the inversion layer to achieve the voltage required during read/programming operation. Moreover, even or odd number of the select gate can be connected together though respective metal line to reduce the N⁺ diffusion and metal line overhead. The implementation of the inverse layer shown in FIG. 6 is only an embodiment, other equivalent manners also can be employed.

Besides the manufacturing method regarding the NMOS type transistor mentioned above, the PMOS type transistor can also be implemented by doping boron ions without departing from the spirit of the present invention.

The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims. 

1. An operation method for a non-volatile memory structure formed between two doping regions serving as bit lines in a semiconductor substrate, the non-volatile memory structure comprising a first conductive line serving as a select gate and being formed above the semiconductor substrate, two conductive blocks serving as floating gates and being formed at the two sides of the first conductive line and insulated from the first conductive line with two first dielectric spacers therebetween, a first dielectric layer formed on the two second conductive blocks, a second conductive line serving as a word line and being formed on the first dielectric layer and substantially perpendicular to the two doping regions; wherein reading the programmed status of one of the conductive blocks comprising the step of putting a bias voltage on the doping region next to the conductive block to be read, a bias voltage on the second conductive line, and a bias voltage on the first conductive line next to the conductive block to be read, so as to turn on the select gate and form an inversion layer underneath the select gate; and the doping region, the channel under the conductive block and the inversion layer under the select gate form a reading path during the reading operation.
 2. The operation method of claim 1, wherein the conductive block is programmed by putting a bias voltage on the doping region next to the conductive block to be programmed, a bias voltage on the second conductive line, and a bias voltage on the first conductive line next to the conductive block to be programmed, so as to turn on the select gate and form an inversion layer underneath the select gate; and the doping region, the channel under the conductive block to be programmed and the inversion layer under the select gate form a programming path during the programming operation.
 3. The operation method of claim 1, wherein the inverse layer is connected to a circuit at a voltage around 0 volts. 